Chassis arrangement for reducing flame emissions

ABSTRACT

A chassis arrangement that minimizes lateral flame spread from card to card within the chassis, and increases heat dissipation from the chassis, comprising a heat sink attached to the inside of the chassis cover to dissipate excess heat from within the chassis, and PCB shields disposed between the circuit cards of the chassis and acting in concert with the heat sink to act as physical and thermal barriers to prevent lateral flame spread from card to card due to flame and hot gas wrap-around.

FIELD OF THE INVENTION

[0001] The present invention relates to chassis for electronicenclosures, and more particularly to an apparatus for reducing flameemissions from such chassis.

BACKGROUND OF THE INVENTION

[0002] For telecommunications equipment installed in central offices(COs) and other telephone buildings, it is recommended that theequipment meet certain minimum equipment-building interface, physicaldimension and environmental performance criteria. One such set ofcriteria is Telcordia standards document GR-63-CORE, “NetworkEquipment-Building System (NEBS) Requirements: Physical Protection,”Issue 1, October, 1995.

[0003] With certain equipment failures, it is possible for somecomponents to overheat to the extent that they ignite a fire. The NEBSstandard includes criteria, described in Section 4.2, that provideminimum fire resistance recommendations for CO telecommunicationequipment. These criteria are designed to minimize the fire propagationhazard, and to minimize the likelihood that an equipment assembly firewill spread beyond the structural elements of the equipment on fire.These criteria address measurable conditions that include ignition ofmaterial in adjacent equipment frames and assemblies, and visible flamesextending beyond the horizontal or vertical confines of the equipment onfire. Compliance to the standard is tested by introducing a flame sourceinto the equipment under test, and observing how the equipment reactsonce ignition has occurred.

SUMMARY OF THE INVENTION

[0004] The present invention is a chassis arrangement that minimizes thelateral flame spread from card to card within the chassis, and increasesheat dissipation from the chassis. The invention comprises a heat sinkattached to the underside of the chassis cover to dissipate excess heatfrom within the chassis, and PCB shields disposed between the circuitcards of the chassis and acting in concert with the heat sink to act asphysical and thermal barriers to prevent lateral flame spread from cardto card due to flame and hot gas wrap-around.

DESCRIPTION OF THE DRAWINGS

[0005] The FIGURE shows an exploded view of a preferred embodiment ofthe present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

[0006] The FIGURE shows an exploded view of a preferred embodiment ofthe present invention. Chassis 1 and chassis cover 2 comprise anequipment enclosure that defines enclosed internal compartment 7 forholding printed circuit board (PCB) assemblies such as PCB 3. A PCBshield 4 is disposed between all circuit boards 3. Heat sink 5 isthermally coupled to the bottom face of chassis cover 2.

[0007] In the preferred embodiment, PCB shields 4 are of approximatelythe same height and depth as PCB cards 3, and are oriented approximatelycoplanar with PCB cards 3. More particularly, chassis 1 and PCB shields4 are dimensioned such that the front and rear edges of PCB shields 4,when PCB shields 4 are installed in chassis 1, are approximately flushwith the front and rear faces of internal compartment 7 in which PCBcards 3 are installed. In the context of the present invention,approximately flush can also mean actually flush. In addition, PCBshields 4 and heat sink 5 are dimensioned such that, when chassis cover2 is installed on chassis 1, the top edges of PCB shields 4 areapproximately flush with the bottom face of heat sink 5. Having anapproximately flush relationship between the front, rear and top edgesof PCB shields 4 and the front and rear faces of internal compartment 7and the bottom face of heat sink 5, respectively, operates to reduce oreliminate lateral flame spread from card to card due to flame and hotgas wrap-around at the edges of PCB cards 3. In addition, PCB shields 4act as a physical and thermal barrier between adjacent PCB cards 3 suchthat a PCB card that is on fire or emitting hot gasses does not exposethe opposing face of an adjacent PCB card to flame, excessive heat orhot gasses, and directs excessive heat towards heat sink 5. Since heat,hot gasses and flames tend to naturally rise, a flush relationshipbetween the bottom edge of PCB shields 4 and the bottom face of internalcompartment 7 is not critical for the reduction of lateral flame spreadfrom card to card due to flame and hot gas wrap-around at the edges ofPCB cards 3.

[0008] In general, the dimensioning of PCB shields 4 and the proximalrelationship of the edges of PCB shields 4 to the faces of internalcompartment 7 and the bottom face of heat sink 5 is such that lateralflame spread from card to card due to flame and hot gas wrap-around atthe edges of PCB cards 3 is greatly reduced or eliminated. For example,if internal compartment 7 is deeper than PCB cards 3, PCB shields 4 mayonly need to extend a small amount beyond the front and/or rear edges ofPCB cards 3 to reduce lateral flame spread. Also, while design of PCBshields 4 to cover as much of PCB cards 3 as possible will tend tominimize lateral flame spread, the positioning of combustible materialand concentration of fuel load on PCB cards 3 may allow PCB shields 4 tobe dimensioned smaller than PCB cards 3 and positioned between PCB cards3 so as to be effective in reducing or eliminating flame spread fromcard to card. Other considerations in the positioning, orientation andthickness of PCB shields 4 include that the shields not adversely affectthe internal normal operating temperature of the equipment assemblybecause of excessive air blockage or air flow impedance.

[0009] In the preferred embodiment, PCB shields 4 comprise 0.019″galvanized sheet steel with a backing of flame retardant non-conductivefiberglass cloth tape. The tape also acts to prevent shorts on the cardduring installation and removal. This combination of materials proves toperform satisfactorily, is easy to fabricate, and is relativelyinexpensive. Other materials may be used that satisfy specific design,performance and economic criteria.

[0010] In the preferred embodiment, PCB shields 4 are attached to PCBcards 3 by screws and standoffs at the rear of the card at card handle6, and at the front of the card. The flame retardant tape is applied tothe face of PCB shield 4 that faces the PCB card to which the shield isattached. The specific means to dispose PCB shields 4 between PCB cards3 is not important as long as the objectives of the present inventionare met.

[0011] Heat sink 5 operates to dissipate heat from within internalcompartment 7 so as to reduce the intensity of flames that may bepresent on PCB cards 3, and to reduce the incidence of flame ignition onPCB cards 3 due to excessive heat build-up in internal compartment 7.

[0012] Heat sink 5 is dimensioned laterally such that it at least spansPCB cards 3 such that excess heat generated by ignition of combustiblematerial on a card can rise and be transferred to heat sink 5. In thepreferred embodiment, heat sink 5 extends approximately the entire widthof chassis cover 2. Generally, where internal compartment 7 may be widerthan the portion reserved for card installation, heat sink 5 need onlyspan sufficiently to dissipate excess heat generated from ignition andflames from PCB cards 3, and also satisfy the desire to reduce card tocard flame spread from flame wrap-around over the top edge of PCB cards3. Thus, heat sink 5 may not need to span the entire width of internalcompartment 7. Similarly, generally, the longitudinal dimension of heatsink 5 need only be sufficient to dissipate excess heat generated fromignition and flames from PCB cards 3, and also satisfy the desire toreduce card to card flame spread from flame wrap-around over the topedge of PCB cards 3. In the preferred embodiment, heat sink 5 extendsfrom the front of internal compartment 7 to the vent holes at the rearportion of chassis cover 2.

[0013] The selection of material for heat sink 5 is based on criteriathat include mass, flammability, thermal conductivity, linear thermalexpansion, cost, availability and ease of fabrication. In the preferredembodiment, 0.3125″ plate aluminum is used. However, any suitablematerial that satisfies the design and selection criteria may be used.

[0014] Heat sink 5 is sufficiently thermally coupled to the bottom faceof chassis cover 2 so as to allow the transmission of heat from heatsink 5 through chassis cover 2 and to the environment. The couplingmeans for attaching heat sink 5 to chassis cover 2 is not, per se,important, and may comprise, for example, screws, rivets, retainingtabs, or thermal coupling adhesive.

[0015] An alternative embodiment for heat sink 5 and chassis cover 2comprises a heat sink 5 having a longitudinal dimension that is shorterthan that of PCB cards 3, and includes non-flammable material, such asfoam or tape, attached to chassis cover 2 adjacent to heat sink 5, andabove and approximately or actually flush with PCB cards 3. In thisarrangement, heat sink satisfies the thermal related criteria, and heatsink 5 and the non-flammable material act in concert to reduce oreliminate lateral flame spread from card to card due to flame and hotgas wrap-around at the top edges of PCB cards 3.

[0016] A second alternative embodiment includes a thermal fuse attachedto any cooling fans in the equipment assembly that may vent hot gassesand flames outside the equipment enclosure. In the event of a fire thatproduces excessive heat within the enclosure, the cooling fans shutdown.

[0017] While the present invention has been shown and described withrespect to exemplary embodiments, it will be understood by those skilledin the art that modifications may be made thereto without departing fromthe scope and spirit of the invention. It is intended that the scope ofthe invention be defined by the claims appended hereto and theirequivalents.

What is claimed is:
 1. A chassis arrangement for reducing flameemissions, a chassis and chassis cover defining an internal compartmenthaving printed circuit board (PCB) cards installed within, said chassisarrangement comprising: a heat sink thermally coupled to the bottom faceof the chassis cover so as to allow the transmission of excess heat fromthe internal compartment through said heat sink and the chassis coverand to the environment; and PCB shields disposed between the PCB cards,said PCB shields acting as physical and thermal barriers betweenadjacent PCB cards, said PCB shields acting in concert with said heatsink to prevent or reduce lateral flame spread from PCB card to PCB carddue to flame wrap-around and to direct excess heat toward said heatsink.
 2. A chassis arrangement according to claim 1, wherein the topedges of said PCB shields are approximately flush with the bottom faceof said heat shield.
 3. A chassis arrangement according to claim 1,wherein the top edges of said PCB shields are approximately flush withthe bottom face of said heat shield, and the front and rear edges ofsaid PCB shields are approximately flush with the front and rear faces,respectively, of the internal compartment.
 4. A chassis arrangementaccording to claim 2, further comprising non-flammable material attachedto the chassis cover, adjacent to said heat sink, and approximatelyflush with the top edges of the PCB cards, said non-flammable materialacting in concert with said heat sink to prevent or reduce lateral flamespread from PCB card to PCB card due to flame wrap-around.
 5. A chassisarrangement according to claim 1, the chassis further comprising aforced air cooling system, said chassis arrangement further comprising athermal fuse acting to shut down the forced air cooling system, therebyreducing the venting of hot gasses or flames outside the chassis.